Conventional holographic polymer dispersed liquid crystal (HPDLC) mediums are formed in a mixture of liquid crystal and a photo-curable monomer. On exposure to a holographic interference pattern, typically formed by coherent lasers, polymerization is initiated, creating a diffusion gradient that in turn causes migration of liquid crystals. Alternating planes of polymer and liquid crystal droplets are formed in the mixture corresponding to the interference pattern, resulting in the formation of a Bragg grating that can reflect a specific wavelength of light. The wavelength of light that will be reflected is determined in part by the incident angle of the laser beams on the mixture during polymerization. Typically, a single Bragg grating is formed, resulting in a narrow peak reflection wavelength, normally in the range of 5 to 20 nm full width at half maximum (FWHM).
A desirable property in HPDLC mediums is the ability to reflect a range of wavelengths of light. Several techniques are typically used to produce this property. In one technique, multiple HPDLC mediums, each reflecting a specific wavelength, are bonded together in a stacked configuration. However, each subsequent HPDLC layer increases the attenuation of light as it passes through stacked HPDLC mediums. The increased attenuation characteristics of stacked HPDLC mediums, combined with the narrow peak reflections of each HPDLC layer, make the manufacture of stacked HPDLC mediums capable of reflecting a broad spectrum of wavelengths difficult and impractical.
Another technique involves the creation of multiple HPDLC Bragg gratings in a single layer through the use of simultaneous, coherent multiple laser beam exposure. Typically, at least two pairs of laser beams are used, with each beam incident on the mixture at a different angle, in order to form an optical interference pattern associated with reflection of a different wavelength of light. This technique may provide HPDLC mediums having a plurality of spectrally overlapping reflectances, giving rise to a range of peak reflected wavelengths within a single medium. However, the range of peak reflected wavelengths results in an HPDLC medium lacking a uniform reflective behavior across a broad range of wavelengths. Additionally, increasing the number of Bragg gratings in a single layer increases the complexity of the hologram fabrication setup and requires additional lasers to maintain a reasonable range of beam irradiance. Despite the availability of the above-discussed techniques, a desire remains for the ability to create HPDLC mediums capable of reflecting a broad range of peak reflected wavelengths.